Systems and methods for a narrow band high transmittance interference filter

ABSTRACT

The present disclosure provides an interference filter, a lithography system incorporating an interference filter, and a method of fabricating an interference filter. The interference filter includes a transparent substrate having a front surface and a back surface, a plurality of alternating material layers formed over the front surface of the transparent substrate that form a bandpass filter, and an anti-reflective structure formed over the back surface of the transparent substrate. The alternating material layers alternate between a relatively high refractive index material and a relatively low refractive index material.

The present application is a divisional application of U.S. patentapplication Ser. No. 13/929,419, filed Jun. 27, 2013, which claims thebenefit of U.S. Provisional Application No. 61/785,488 filed Mar. 14,2013, entitled “Systems and Methods for a Narrow Band High TransmittanceInterference Filter,” each of which is incorporated herein by referencein its entirety.

BACKGROUND

The semiconductor integrated circuit industry has experienced rapidgrowth in the past several decades. Technological advances insemiconductor materials and design have produced increasingly smallerand more complex circuits. These material and design advances have beenmade possible as the technologies related to processing andmanufacturing have also undergone technical advances. In the course ofsemiconductor evolution, the number of interconnected devices per unitof area has increased as the size of the smallest component that can bereliably created has decreased.

The pursuit of smaller feature size has required a number oftechnological changes, including changes to lithography systems.Lithography systems include a light or radiation source that isfiltered, directed, and focused in order to better transmit a patternfrom a mask onto a wafer. One trend of recent years has been to adjustthe wavelength of light used to expose semiconductor wafers. Anothertrend has been to use phase-shifting masks. However, despite theseimprovements in lithography systems, the current technology has not beenentirely satisfactory.

BRIEF DESCRIPTION OF THE DRAWINGS

It is emphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIG. 1 is a diagram of a lithography system incorporating aninterference filter according to an embodiment of the presentdisclosure.

FIG. 2 is a diagram illustrating the effects of an interference filteron light coming from a source according to an embodiment of the presentdisclosure.

FIGS. 3A, 3B, 3C, 3D, and 3E are fragmentary cross-sectional views ofthe fabrication of an interference filter according to an embodiment ofthe present disclosure.

FIG. 4 is a fragmentary cross-sectional view of an interference filteraccording to an alternative embodiment of the present disclosure.

FIG. 5 is a flowchart of a method for fabrication an interference filteraccording to an embodiment of the present disclosure.

Aspects of the figures in the present disclosure are best understoodfrom the following detailed description when read in connection with thefigures.

DETAILED DESCRIPTION

It is to be understood that the following disclosure provides manydifferent embodiments, or examples, for implementing different featuresof the invention. Specific examples of components and arrangements aredescribed below to simplify the present disclosure. These are, ofcourse, merely examples and are not intended to be limiting. Moreover,the formation of a first feature over or on a second feature in thedescription that follows may include embodiments in which the first andsecond features are formed in direct contact, and may also includeembodiments in which additional features may be formed interposing thefirst and second features, such that the first and second features maynot be in direct contact. Further still, references to relative termssuch as “front” and “back” are used to provide a relative relationshipbetween elements and are not intended to imply any absolute direction.Various features may be arbitrarily drawn in different scales forsimplicity and clarity.

FIG. 1 is a diagrammatic representation of a lithography tool or system100 according to an embodiment. Lithography system 100 is simplified ina number of respects for clarity of explanation. Lithography system 100includes a light source 102. Light source 102 serves as a source oflight or radiation in photolithographic processes such as may beperformed during semiconductor device fabrication. In the depictedembodiment, light source 102 is a mercury lamp, while in otherembodiments light source may have a different type of lamp or source.The light generated by light source 102 travels a path illustrated inFIG. 1 by dashed-line arrows. To conserve space in a fabricationfacility, the path of lithography system 100 may have a number of bends.These bands may be provided by mirrors, prisms, and/or other opticdevices. The first bend occurs in the depicted light path when lightfrom light source 102 encounters a first mirror 104. The first mirror104 directs the light through a number of filters, lenses, and otheroptical devices. A first filter 106 may include a plurality of filtersor may be a single filter. For example, first filter 106 may include anultra-violet block filter and/or a WG filter. Lithography system 100further includes a shutter unit 108 that may be used to selectively stopor permit the transmission of light to a target.

After the shutter unit 108 is an interference filter 110. In thedepicted embodiment interference filter 110 includes a bandpass filterthat permits a narrow band of wavelengths centered on or around adesired central wavelength to pass through the filter while rejectingother wavelengths. The lithography system 100 of FIG. 1 also includes anintegrator block 112 before a second mirror 114, which causes anotherbend in the path of light coming from light source 102. Embodiments oflithography system 100 further include a gradient filter 116, acondenser lens 118, a reticle masking system 120, and a chuck 122 uponwhich a wafer to be exposed is positioned. Embodiments of lithographysystem 100 may contain a number of additional components not discussedabove, such as a zoom lens, a field lens, an energy sensor, and a spotsensor, etc. These additional components may be apparent to one of skillin the art and are outside the scope of this disclosure. Furthermore,the components of lithography system 102 discussed above may be arrangedin various sequences and a number of configurations that are within thescope of this disclosure.

FIG. 2 illustrates the effects of interference filter 110 on the lightreceived from light source 102. Light source 102 generates an unfilteredspectrum 202 of light having a plurality of peaks or lines depending onthe type of light source used as light source 102. Several peaks or“lines” are depicted in the unfiltered spectrum 202 of FIG. 2, includingi-line 204, h-line 206, and g-line 208. Each of these alphabeticallydesignated lines represents a particular wavelength or small set ofwavelengths provided by the spectrum of light source 102. In thisexample, i-line 204 is a peak around 370 nm, h-line 206 represents apeak around 405 nm, and g-line 208 is a peak around 440 nm.

It is desirable that interference filter 110 pass light in a narrowrange of wavelengths, or have a narrow full wavelength of half maximum(FWHM), have a high transmission rate around a desired centralwavelength, and a very low transmission rate away from the centralwavelength. As depicted in FIG. 2, after light from light source 102passes through interference filter 110, its filtered spectrum 210 is asdepicted. Of note, i-line 204 is transmitted through interference filter110, while h-line 206 and g-line 208 are blocked. While a peaktransmission rate of interference filters may typically be around 80%,the peak transmission rate of interference filter 110 may be closer to90 or 95%. As will be discussed in more detail below, interferencefilter 110 includes a transparent substrate with a plurality of materiallayers deposited on either surface of the transparent substrate. Theplurality on a front side of interference filter 110 acts as a bandpassfilter, while the plurality on the back side acts as an anti-reflectivestructure. Interference filter 110 may be orientated within lithographysystem 100 so that either the bandpass filter side of theanti-reflective structure side is closest to light source 102.

FIG. 3A depicts an interference filter 300 such as may be used asinterference filter 110 of the lithography system 100 depicted inFIG. 1. Fabrication of interference filter 300 begins with a transparentsubstrate 302. The substrate 302 is transparent with respect to thelight of the desired narrow range of light provided by an applied sourcesuch as light source 102. Thus, embodiments of substrate 302 may betransparent with respect to a given wavelength but not as transparentwith respect to a different wavelength. As depicted, substrate 302 is afused quartz substrate, with a first or front surface 304 and a secondor back surface 306. Other materials may be used for the transparentsubstrate 302 in other embodiments. At this stage of fabrication, theremay be no significant difference between front surface 304 and backsurface 306. As depicted, substrate 302 has undergone one or morepolishing steps or processes to improve both of surfaces 304 and 306.

FIG. 3B further depicts a first material layer 310 over the frontsurface 304 of substrate 302. First material layer 310 is largelytransparent with respect to the desired wavelength. Of particularimportance, the first material layer 310 has a first refractive index,and is fabricated in such a way that the mathematical product of thefirst refractive index and a thickness of the first material layer 310are equal to a quarter of the desired central wavelength to be passed byinterference filter 300. In the depicted embodiment, the thickness issuch that the product is equal to a quarter of the desired wavelength.Using a quarter wavelength allows the first material layer 310 to bethinner than if a half wavelength thickness were used.

A number of materials may be used for first material layer 310. Asdepicted, the first material layer 310 is formed from MgF₂ and has arefractive index of 1.38 and a film thickness of 66.12 nm. In thisexample, a desired wavelength or central wavelength of interferencefilter 300 is around 370 nm, i.e. 1.38×66.12 nm×4 equals around 370 nm.In other embodiments, first material layer 310 may be one of TiO₂,Al₂0₃, Ta₂0₅, and SiO₂, or another material. Example thicknesses andrefractive indices for these materials is included in Table 1 below fora desired a central wavelength of around 370 nm.

TABLE 1 Material Refractive Index Thickness (nm) MgF₂ 1.38 66.12 TiO₂2.35 38.83 Al₂0₃ 1.6 57.03 Ta₂0₅ 2.2 41.48 SiO₂ 1.47 62.07

First material layer 310 may be formed by a plurality of materialdeposition processes such as chemical vapor deposition, physical vapordeposition, atomic layer deposition, and other material depositionprocesses. In the depicted example, first material layer 310 is formedusing a radiofrequency-driven ion beam assisted deposition process.First material layer 310 is formed so that it is an amorphous filmlayer, rather than a polycrystalline film layer. This may enhanceresistance to environmental factors and reduce scattering of lighttransmitted through interference filter 300. In some embodiments, afterfirst material layer 310 is deposited a polishing process may beperformed. In some other embodiments, an annealing process may beperformed.

FIG. 3C is also a fragmentary cross-sectional diagram of interferencefilter 300 while in the process of fabrication. As depicted in FIG. 3C,a second material layer 312 is deposited over the first material layer310. Similar techniques may be used in the deposition of second materiallayer 312 as those used in the deposition of first material layer 310.For example second material layer 312 may be deposited in an ion beamassisted deposition process. Second material layer 312 is formed from adifferent material than that used to form first material layer 310. Thematerial of second material layer 312 and the material of first materiallayer 310 are selected such that one material has a relatively highrefractive index while the other material has a relatively lowrefractive index.

Continuing the example in which the first material layer 310 is formedfrom MgF₂, second material layer 312 is formed from TiO₂, has arefractive index of 2.35, and is 38.83 nm thick. In this example, therefractive index of first material layer 310 (1.38) is relatively lowcompared to the refractive index of the second material layer 312(2.35). It may be useful to think of first material layer 310 and secondmaterial layer 312 as a pair of layers 314.

FIG. 3D depicts interference filter 300 with a plurality of pairs oflayers 314 deposited on front surface 304 over substrate 302, in alow-high-low-high pattern. FIG. 3D explicitly depicts four such pairs oflayers 314, but impliedly depicts many more. In general, front surface304 has an even number of alternating material layers deposited over it.Embodiments of filter 300 may include a low number of material layers ofabout 20 layers to a high number of material layers at about 40 layers.In other words, front surface 304 may have from about 10 to about 20pairs of layers to 14 deposited thereon. Some embodiments may have moreor fewer. The plurality of alternating material layers deposited overfront surface 204 forms a bandpass filter 316 that has a peaktransmission at the desired wavelength, e.g. 370 nm. The depictedembodiment of interference filter 300 includes a bandpass filter 316having a first material layer 310 formed from the relatively lowrefractive index material, and the second material layer 310 formed fromthe relatively high refractive index material. In some embodiments,first material layer 310 is formed from a relatively high refractiveindex material, and second material layer 312 is formed from arelatively low refractive index material as will be discussed in FIG. 4.

FIG. 3E depicts interference filter 300 with an anti-reflectivestructure 318 formed over back surface 306. Anti-reflective structure318 may improve the transmittance of interference filter 300.Anti-reflective structure 318 includes a plurality of alternatingmaterial layers, similar to the alternating layers (first material layer310, then second material layer 312) used to form the bandpass filter316 over front surface 304. Like bandpass filter 316, anti-reflectivestructure 318 includes a plurality of alternating material layers thatalternate between a relatively high refractive index layer and arelatively low refractive index layer. As depicted in FIG. 3E,anti-reflective structure 318 includes a first anti-reflective layerformed from a relatively high refractive index material, and a secondanti-reflective layer formed thereover from a relatively low refractiveindex material. In accord with the example provided above, the firstanti-reflective layer is formed from MgF₂ and the second anti-reflectivelayer is formed from TiO₂. The alternating material layers ofanti-reflective structure 318 may be formed by the same process as isused to form the first material layer 310 and second material layer 312and the additional material layers formed of thereover. In the depictedembodiment, the alternating material layers of anti-reflective structure318 are amorphous material layers formed using an ion beam assisteddeposition technique using a high bias voltage.

Expressly depicted in FIG. 3E are three layers in anti-reflectivestructure 318. More than three layers are impliedly depicted. Forexample, some embodiments of interference filter 300 include fivealternating material layers in anti-reflective structure 318.Anti-reflective structure 318 may include from about three to more thaneight alternating material layers. In some embodiments, anti-reflectivestructure 318 includes an even number of alternating material layers,while in other embodiments it contains an odd number of alternatingmaterial layers. Additionally, while in some embodiments the materiallayer of anti-reflective structure 318 that is closest to substrate 302is made from a relatively high refractive index material, in otherembodiments the material layer of anti-reflective structure 316 closestto substrate 302 is made from a relatively low refractive indexmaterial.

FIG. 4 depicts an interference filter 400 that may be understood as analternative or additional embodiment of interference filter 300 anduseful as interference filter 110 of lithography system 100 of FIG. 1.Interference filter 400 includes a transparent substrate 402, which hasa front side 404 and a back side 406. A plurality of alternatingmaterial layers is deposited on front surface 404 to form a bandpassfilter 416, which is similar to bandpass filter 316 as depicted in FIGS.3D and 3E. Bandpass filter 416 includes a plurality of pairs ofalternating material layers, like pair of layers 414. A pair of layers414 includes a first material layer 410 which is closest to the frontsurface 404 of substrate 402. In the depicted embodiment first materiallayer 410 is formed from a relatively high refractive index material. Asecond material layer 412 is deposited over the first material layer 410and is formed from a relatively low refractive index material. Thus incontrast with interference filter 400 of FIGS. 3D and 3E, which hadalternating material layers arranged in a low-high-low-high patternbeginning over the front surface 404 of substrate 402, interferencefilter 400 has alternating material layers arranged in ahigh-low-high-low pattern to form bandpass filter 408.

In the depicted embodiment of interference filter 400, first materiallayer 420 is formed from Ta₂0₅, which has a refractive index of 2.2. Fora desired central wavelength of 370 nm, first material layer 410 isabout 41.48 nm thick. Second material layer 412 is formed from SiO₂,which has a refractive index of 1.47. Thus, for the desired centralwavelength of 370 nm, second material layer 414 is about 62.07 nm thick.

An anti-reflective structure 418 is formed over back surface 406. Likebandpass filter 416, anti-reflective structure 418 includes a pluralityof alternating material layers. Unlike anti-reflective structure 318 ofFIGS. 3D and 3E, which had a high-low-high-low pattern moving away fromsubstrate 302, anti-reflective structure 418 has a low-high-low-highpattern. In the depicted embodiment, anti-reflective structure 418includes the same materials used to form bandpass filter 416. Thus thefirst of the alternating material layers of anti-reflective structure418 is formed from SiO₂, and the layer formed thereover is formed fromTa₂0₅. In some embodiments, bandpass filter 416 may be formed fromalternating layers of a first and a second material, whileanti-reflective structure 418 is formed from alternating layers of athird and a fourth material. In the depicted embodiment, the layer ofbandpass filter 416 and the layer of anti-reflective structure 418 thatare closest to substrate 402 are opposite, i.e. one is a relatively highrefractive index material while the other is a relatively low refractiveindex material. In some embodiments, both bandpass filter 416 andanti-reflective structure 418 include the same material in the layer ofeach that is closest to substrate 402.

FIG. 5 is a flowchart of a method 500 for fabricating an interferencefilter. As depicted, method 500 includes a plurality of steps. Manyembodiments of method 500 may be apparent to one of skill in the artthat have additional steps before and/or after any or all of thedepicted steps. Such embodiments are within the scope of thisdisclosure. Method 500 may begin in step 502 when a transparentsubstrate with a first surface and a second surface is positioned withina processing system. In step 504, the processing system is used todeposit a plurality of alternating material layers over the firstsurface of the transparent substrate. In step 506, the same processingsystem or a different processing system is used to deposit ananti-reflective structure over the second surface of the transparentsubstrate. The second surface is opposite the first surface relative tothe transparent substrate. For example, the first surface may be a frontsurface while the second surface is a back surface of the transparentsubstrate.

For clarity of explanation, reference is made to FIGS. 3A, 3B, 3C, 3D,and 3E, and the corresponding descriptions above to more clearly explainmethod 500. In performing method 500, a desired wavelength to be passedby an interference filter, like interference filter 500, may bedetermined or selected. For example, the desired wavelength, which isthe central wavelength of interference filters 110, 300, and 400, maybeabout 370 nm. The substrate 302 is positioned with a deposition chamber(step 502). A first material layer 310 is deposited over the frontsurface 304 of substrate 302. The first material layer 310 may have arelatively low refractive index, and may be deposited as an amorphouslayer by ion beam assisted deposition. The second material layer 312having a relatively high refractive index, is deposited over the firstmaterial layer 310. After the high refractive index material layer isdeposited, another low refractive index material layer is depositeduntil there are about 30 alternating material layers in alow-high-low-high pattern (step 504) to form a bandpass filter 316.

Depositing the anti-reflective structure 318 over the second surface ofthe transparent substrate includes depositing a plurality of alternatingmaterial layers, wherein the materials alternate from a relatively highrefractive index material layer to a relatively low refractive indexmaterial layer or from a relatively low refractive index material layerto a relatively high refractive index material layer (step 506).

One embodiment of the invention is an optical component such as aninterference filter. The interference filter includes a transparentsubstrate having a front surface and a back surface. A plurality ofalternating material layers are formed over the front surface of thetransparent substrate, with an anti-reflective structure formed over theback surface of the transparent substrate. In some embodiments, theplurality of alternating material layers includes a plurality of pairsof alternating material layers, with each pair of alternating materiallayers including a first material layer and a second material layer. Thefirst material layer has a higher refractive index than the secondmaterial layer.

Each of the plurality of alternating material layers has a refractiveindex and a thickness. And the refractive index and thickness of eachmaterial layer are determined by the wavelength of light to be passed bythe interference filter. Embodiments of the interference filter includeabout 20 about 40 material layers in the plurality of alternatingmaterial layers. In at least some embodiments, each of the plurality ofalternating material layers has a refractive index and a thickness, anda quarter of a center wavelength of the interference filter is equal tothe refractive index multiplied by the thickness of each of theplurality of alternating material layers.

The plurality of alternating material layers is formed by ion beamassisted deposition in some embodiments, and are each an amorphous filmlayer. The alternating material layers in the interference filter areformed from two of MgF₂, TiO₂, Al₂0₃, Ta₂0₅, and SiO₂. In someembodiments, other materials are selected. The plurality of alternatingmaterial layers has an even number of material layers and theanti-reflective structure has an odd number of material layers, in someembodiments. Further, in some embodiments the plurality of alternatingmaterial layers has layers of a first material and layers of a secondmaterial, and the anti-reflective structure comprises a plurality oflayers of the same first material and the same second material.

Another embodiment is a lithography system that includes a light sourcewith a plurality of spectrum peaks in the light it emits, a plurality oflenses, at least one mirror and a plurality of filters including aninterference filter. The interference filter has a plurality ofalternating material layers deposited on a substrate, and allows one ofthe plurality of spectrum peaks to pass, while blocking the rest of theplurality of spectrum peaks. In some related embodiments, the pluralityof alternating material layers of the interference filter includeslayers of a first material and layers of a second material, where thefirst material has a higher refractive index than that of the secondmaterial. Each of the plurality of material layers is an amorphous filmlayer.

In some embodiments, interference filter in the lithography systemfurther includes an anti-reflective structure deposited on thesubstrate. The anti-reflective structure and the plurality ofalternating material layers are deposited on opposite sides of thesubstrate. In the interference filter, each of the plurality ofalternating material layers has a thickness corresponding to a quarterwavelength of a desired center wavelength.

Another embodiment is a method for fabricating an interference filter.Embodiments of the method include steps of positioning a transparentsubstrate having a first surface and a second surface in a processingtool, depositing a plurality of alternating material layers over thefirst surface of the transparent substrate, and depositing ananti-reflective structure over the second surface of the transparentsubstrate. The second surface is opposite the first surface relative tothe transparent substrate. Some embodiments of the method furtherinclude a step of determining a desired center wavelength to be passedby the interference filter. Depositing the plurality of alternatingmaterial layers includes depositing a first material having a lowrefractive index and depositing a second material having a highrefractive index.

In some related embodiments, depositing the anti-reflective structureincludes depositing a plurality of material layers that alternatebetween a first material and a second material. Depositing the pluralityof alternating material layers over the first surface includesdepositing a plurality of amorphous material layers.

What is claimed is:
 1. A method for forming an interference filter,comprising: forming a substrate; forming a first plurality of an oddnumber of material layers alternating between first and secondrefractive indices over a first side of the substrate, wherein formingthe first plurality of material layers comprises: forming a firstamorphous material layer that physically contacts the first side of thesubstrate; forming a second amorphous material layer, wherein the firstamorphous material layer has a higher refractive index than the secondamorphous material layer; and forming a fifth amorphous material layerfurther away from the first side of the substrate than any othermaterial layers of the first plurality of material layers, wherein thefirst and fifth amorphous material layers have a same thickness measuredin a direction perpendicular to the first side of the substrate; andforming a second plurality of an even number of material layersalternating between the first and second refractive indices over asecond side of the substrate that is opposite the first side, whereinforming the second plurality of material layers comprises: forming athird amorphous material layer that physically contacts the second sideof the substrate, the third amorphous material layer having the samerefractive index as the second amorphous material layer; forming afourth amorphous material layer disposed over the third amorphousmaterial layer, wherein the third amorphous material layer has a lowerrefractive index than the fourth amorphous material layer; and forming asixth amorphous material layer positioned further away from the secondside of the substrate than any other amorphous material layer of thesecond plurality of material layers, the first amorphous material layerhaving the same refractive index as the fourth, fifth, and sixthamorphous material layers.
 2. The method of claim 1, wherein the sixthamorphous material layer is formed from the same material as the fourthamorphous material layer.
 3. The method of claim 1, wherein the secondplurality of material layers includes from about 20 to about 50 materiallayers.
 4. The method of claim 1, wherein each layer of the first andsecond pluralities of material layers has a refractive index and athickness, and wherein a quarter of a center wavelength of theinterference filter is equal to the refractive index times the thicknessof each layer of the first and second pluralities of material layers. 5.The method of claim 1, further comprising forming one or more layers ofthe first and second pluralities of material layers by ion beam assisteddeposition.
 6. The method of claim 1, wherein the first plurality ofmaterial layers comprises a plurality of amorphous film layers.
 7. Themethod of claim 1, wherein the first plurality of material layers has anodd number of material layers and the second plurality of materiallayers has an even number of material layers greater than the odd numberof material layers.
 8. A method for forming a device, comprising:forming a substrate having a first side and a second side opposite thefirst side; forming a first plurality of material layers on the firstside of the substrate, alternating between first and second refractiveindices, wherein the first plurality of material layers includes a firstamorphous material layer physically contacting the first side of thesubstrate and a second amorphous material layer, wherein the firstamorphous material layer has a higher refractive index than the secondamorphous material layer, wherein the first plurality of material layersfurther includes a fifth amorphous material layer having the samerefractive index as the first amorphous material layer and is positionedfurther away from the first side of the substrate than any othermaterial layers from the first plurality of material layers, wherein thefirst and fifth amorphous material layers have a same thickness measuredin a direction perpendicular to the first side of the substrate; andforming a second plurality of material layers over the second side ofthe substrate, alternating between the first and second refractiveindices, the second plurality of material layers including a thirdamorphous material layer physically contacting the second side of thesubstrate and a fourth amorphous material layer disposed over the thirdamorphous material layer, the third amorphous material layer having alower refractive index than the fourth amorphous material layer, whereinthe second plurality of material layers further includes a sixthamorphous material layer having the same refractive index as the first,fourth, and fifth amorphous material layers and is positioned furtheraway from the second side of the substrate than any other amorphousmaterial layer forming the second plurality of material layers, whereinthe second and third amorphous material layers have the same refractiveindex.
 9. The method of claim 8, wherein each layer of the first andsecond pluralities of material layers is an amorphous film layer. 10.The method of claim 8, further comprising forming the first amorphousmaterial layer with the same material as the fourth amorphous materiallayer.
 11. The method of claim 8, further comprising forming the secondamorphous material layer with the same material as the third amorphousmaterial layer.
 12. The method of claim 8, wherein the first pluralityof material layers has an odd number of material layers and the secondplurality of material layers has an even number of material layers. 13.The method of claim 8, wherein the first amorphous material layer isformed of TiO₂ and the second amorphous material layer is formed ofMgF₂.
 14. The method of claim 8, wherein the second amorphous materiallayer is formed of the same material as the third amorphous materiallayer.
 15. A method for forming a device, comprising: forming asubstrate; forming a first amorphous material layer with a firstrefractive index that physically contacts a first side of the substrate;forming a second amorphous material layer with a second refractive indexlower than the first refractive index over the first amorphous materiallayer; forming a third amorphous material layer with a third refractiveindex that physically contacts a second side of the substrate oppositethe first side; forming a fourth amorphous material layer with a fourthrefractive index higher than the third refractive index over the thirdamorphous material layer; forming a fifth amorphous material layerfurther away from the first side of the substrate than the first orsecond amorphous material layers, wherein the first and fifth amorphousmaterial layers have a same thickness measured in a directionperpendicular to the first side of the substrate; and forming a sixthamorphous material layer positioned further away from the second side ofthe substrate than the third or fourth amorphous material layers,wherein the sixth amorphous material layer has the same refractive indexas the first, fourth, and fifth amorphous material layers, wherein thesecond and third amorphous material layers have the same refractiveindex.
 16. The method of claim 15, further comprising a seventhamorphous material layer positioned between the fourth and sixthmaterial layers that has the same refractive index as the secondamorphous material layer.
 17. The method of claim 15, wherein the sixthamorphous material layer is formed of TiO₂.
 18. The method of claim 15,further comprising forming one or more of the first and second amorphousmaterial layers by ion beam assisted deposition.
 19. The method of claim15, further comprising forming one or more of the third and fourthamorphous material layers by ion beam assisted deposition.